1. Field of the Invention
The invention relates to a laser light detection circuit that outputs an electric signal corresponding to the intensity of laser light.
2. Description of the Related Art
In recent years, optical discs such as CD (the abbreviation of a compact disc) is rapidly prevailing and leading in the multimedia age. A Blu-ray Disc using a blue-violet semiconductor laser is developed as a new generation of optical disc.
FIG. 6 is a schematic view showing a structure of an optical disc device 100. The optical disc device 100 includes a semiconductor laser 1, a half mirror 2, a laser light detection circuit 3, a laser driver 4, a microcomputer 5, an optical disc 6 and a data readout device 7.
Laser light generated by the semiconductor laser 1 is reflected by the half mirror 2 and travels to the surface of the optical disc 6. The reflected light from the surface of the optical disc 6 is received by the data readout device 7 through the half mirror 2. The data readout device 7 reads data stored in the optical disc 6 based on the received reflected light.
On the other hand, the laser light generated by the semiconductor laser 1 is received by the laser light detection circuit 3 through the half mirror 2. The laser light detection circuit 3 is a circuit that outputs an electric signal corresponding to the intensity of laser light. In this case, the laser light detection circuit 3 outputs a pair of differential voltage signals, i.e., a first output voltage Vop and a second output voltage Von. The second output voltage Von corresponds to an inverted voltage of the first output voltage Vop relative to a reference voltage.
The laser driver 4 is a circuit that receives the first and second output voltages Vop, Von and controls the intensity of laser light of the semiconductor laser 1 corresponding to the voltage difference between these (Vop−Von). By the feedback control of the laser driver 4, the intensity of laser light generated by the semiconductor laser 1 is controlled so as to be constant. This kind of optical disc device 100 is described in the Japanese Patent Application No. 2003-141767.
The optical disc device 100 having two modes of an operation stop mode (sleep mode) and an operation mode (active mode) has been developed. In this case, the microcomputer 5 outputs a mode switch signal for controlling switching between the operation stop mode and the operation mode. The laser light detection circuit 3 is set to either the operation stop mode or the operation mode in response to the mode switch signal from the microcomputer 5.
However, as shown in FIG. 4, when the laser light detection circuit 3 switches from the operation stop mode to the operation mode, there occurs a problem that the first output voltage Vop of the laser light detection circuit 3 transiently increases to near the supply voltage Vcc (a peak output). (see a chain line curve in FIG. 4)
The laser driver 4 in the next stage to the laser light detection circuit 3 thus receives this peak output as an input voltage. Then the input voltage of the laser driver 4 exceeds the absolute maximum rating, and this may cause the breakdown or malfunction of the laser driver 4. This problem may occur when the supply voltage of the laser light detection circuit 3, e.g. 5V is higher than the supply voltage of the laser driver 4, e.g. 3.3V.
The invention is to address the problem described above, and is directed to preventing a peak output occurring when a circuit switches between the operation stop mode and the operation mode so as to prevent the breakdown or malfunction of the next-connected circuit.